The invention relates to a digital frequency generation device with an output stage and a first digital frequency generator, which generates at its output a first signal with a selectable first frequency f.sub.1 and with a first phase position .sub.1, which can be connected to the output stage of the frequency generating device in such a manner that at the output of the output stage a signal with the first frequency f.sub.1 and the first phase position .sub.1 is present, wherein at any time the first frequency f.sub.1 can be switched over at the output of the output stage to a selectable second frequency f.sub.2 with a phase continuation which is defined with respect to the first frequency f.sub.1, in particular a continuous phase continuation.
A frequency generating device of this type is known for example from the manual "PTS Frequency Synthesizers" by the company Programmed Test Sources Inc. of 1988.
Digital frequency generating devices (frequency synthesizers) in general have the task to provide at their output frequency signals with a selectable frequency of high accuracy and stability. As a rule these are signals with a sine-shaped course of amplitude. Typical applications for such frequency signals are in the area of communications technology, the generation and processing of radar pulses and radar echos, respectively, automatic test systems for the supervision of the stability of time-constant or time-variant quantities and the vast area of the generation and analysis of frequency spectra.
The switching of the output signal of a digital frequency generating device according to the prior art from a first selected frequency to a second, is performed normally in a phase-continuous manner, i.e. at the point in time of switching, the amplitude and phase of the output signal with the first frequency f.sub.1 coincide with amplitude and phase of the output signal at the second frequency f.sub.2. If the output signal is switched back from the frequency f.sub.2 to the frequency f.sub.1, the output signal is also continued in a phase-continuous manner with the actual amplitude and phase value of the f.sub.2 -signal at the point in time of switching back to the frequency f.sub.1. Since the original phase of the first output signal with the frequency f.sub.1 is not stored and when switching back from the frequency f.sub.2 to the frequency f.sub.1, the phase present at the moment is continued, the phase coherence with the original signal is lost in principle in the case of multiple frequency switchings with a conventional digital frequency generating device.
In the manual mentioned above the principle possibility of a phase coherent switching is indicated, in which for example after a switching sequence of the output signal from an original frequency f.sub.1 to a further frequency f.sub.2 and again back to the frequency f.sub.1 the new signal with the frequency f.sub.1 is continued in a phase-coherent manner to the original signal with the frequency f.sub.1, however, this possibility is provided only for switchings of frequency steps of more than 1 MHz, wherein the corresponding frequency shifts between the selected frequencies are generated not with digital, but directly-analogue technology by mixing fixed phase-rigid oscillator frequencies from a system clock.
In the case of many technical applications, in particular in applications on the field of nuclear magnetic resonance it is desired when switching back from the second selected frequency to the first frequency to obtain an output signal, which has the same phase position as the original signal would have at the first frequency at the point in time of switching back, if no switching had occured in the meantime, but wherein the frequency shifts when switching are far below the 1 MHz area such that the application of the described direct-analog mixing technique is not applicable.